Most α/β titanium alloys show superplasticity, i.e., elongation larger than 500%, at sub-transus temperatures when deformed with slower strain rates. The temperature and the strain rate at which superplasticity occurs vary depending on alloy composition and microstructure(1). An optimum temperature for superplastic forming (SPF) ranges from 1832° F. (1000° C.) to as low as 1382° F. (750° C.) in α/β titanium alloys(2). SPF temperatures and beta transus temperatures show a fairly good correlation if other conditions are the same(2).
On the production side, there are significant benefits arising from lowering SPF temperatures. For example, lowering the SPF temperature can result in a reduction in die costs, extended life and the potential to use less expensive steel dies(7). Additionally, the formation of an oxygen enriched layer (alpha case) is suppressed. Reduced scaling and alpha case formation can improve yields and eliminate the need for chemical milling. In addition, the lower temperatures may suppress grain growth thus maintaining the advantage of finer grains after SPF operations(8,9).
Grain size or particle size is one of the most influential factors for SPF since grain boundary sliding is a predominant mechanism in superplastic deformation. Materials with a finer grain size decrease the stress required for grain boundary sliding as well as SPF temperatures(2-4). The effectiveness of finer grains in lowering SPF temperatures was previously reported in Ti-6Al-4V and other alloys(5,6).
There are two approaches for improving superplastic formability of titanium alloys. The first approach is to develop a thermo-mechanical processing that creates fine grains as small as 1 to 2 μm or less to enhance grain boundary sliding. Deformation at lower temperature than conventional hot rolling or forging was studied and an SPF process was developed for Ti-64(5,6).
The second approach is to develop a new alloy system that shows superplasticity at a lower temperature with a higher strain rate. There are several material factors that enhance superplasticity at lower temperatures(1), such as (a) alpha grain size, (b) volume fraction and morphology of two phases, and (c) faster diffusion to accelerate grain boundary sliding(11,16). Therefore, an alloy having a lower beta transus has a potential to exhibit low temperature superplasticity. A good example of an alloy is SP700 (Ti-4.5Al-3V-2Mo-2Fe) that exhibits superplasticity at temperatures as low as 1400° F. (760° C.)(8). FIG. 1 shows the relationship between beta transus and reported SPF temperatures(1,7,9,12,16-20). As a general trend, low beta transus alloys exhibit lower temperature superplasticity. Since Ti-54M has lower beta transus and contains Fe as a fast diffuser, it is expected that the alloy exhibits a lower temperature superplasticity with a lower flow stress than Ti-64. Thus, it may be possible to achieve satisfactory superplastic forming characteristics at low temperature in this alloy without resorting to special processing methods necessary to achieve very fine grain sizes.
Ti-6Al-4V (Ti-64) is the most common alloy in practical applications since the alloy has been well-characterized. However, Ti-64 is not considered the best alloy for SPF since the alloy requires higher temperature, typically higher than 1607° F. (875° C.), with slow strain rates to maximize SPF. SPF at a higher temperature with a lower strain rate results in shorter die life, excessive alpha case and lower productivity.
Ti-54M, developed at Titanium Metals Corporation, exhibits equivalent mechanical properties to Ti-6Al-4V in most product forms. Ti-54M shows superior machinability, forgeability, lower flow stress and higher ductility to Ti6Al-4V(10). In addition, it has been reported that Ti-54M has superior superplasticity compared to Ti-6Al-4V, which is the most common alloy in this application(2). This result is due partly to chemical composition of the alloy as well as a finer grain size which is a critical factor that enhances superplasticity of titanium materials.(21) 
The conventional processing method of titanium alloys is shown in FIG. 2A. First, sheet bar is hot rolled to intermediate gages after heating at about 1650° F. (900° C.) to about 1800° F. (982° C.). Typical gages of intermediate sheets are about 0.10″ to about 0.60″. The intermediate sheets are then heated to about 1650° F. (900° C.) to about 1800° F. (982° C.), followed by hot rolling to final sheets. Typical gages of final sheets are about 0.01″ (0.25 mm) to about 0.20″ (5 mm). Upon final hot cross-rolling, sheets may be stacked in steel pack to avoid excessive cooling during rolling. After rolling to final gage, the sheets are annealed at about 1300° F. (704° C.) to about 1550° F. (843° C.) followed by air cooling. The last stage of the process is to grind and pickle surface to remove alpha case on the surface formed during thermo-mechanical processing.
A method for manufacturing thin sheets of high strength titanium alloys (primarily for Ti6Al-4V) was previously studied by VSMPO in U.S. Pat. No. 7,708,845 and is shown in FIG. 2B.(22) U.S. Pat. No. 7,708,845 requires hot rolling at very low temperatures to obtain fine grains to achieve low temperature superplasticity. The method disclosed in U.S. Pat. No. 7,708,845 can be achieved with rolling mills with very high power, which often lacks flexibility to meet the requirement of a small lot with a variety of gages.(22) The process described in U.S. Pat. No. 7,708,845 is given in the figure as a comparison. In U.S. Pat. No. 7,708,845, rolling is performed at very low temperatures, which may cause excessive mill load, therefore limit the applicability.
Thus, there is a need in the industry to provide a new method for manufacturing titanium alloys that has greater applicability compared to the conventional and prior art methods.